Hcompress Image
Compression Software
Hcompress is the image compression package written by Richard
L. White for use at the Space Telescope Science Institute.
Hcompress was used to compress the
STScI Digitized Sky
Survey and has also been used to compress the preview images in the
Hubble Data Archive.
Briefly, the method used is:
-
a wavelet transform called the H-transform (a Haar transform
generalized to two dimensions), followed by
-
quantization that discards noise in the image while retaining
the signal on all scales, followed by
-
quadtree coding of the quantized coefficients.
The technique gives very good compression for astronomical images and
is fast, requiring about 1 second for compression or decompression of
a 512x512 image on a Sun SPARCstation 2. The calculations are carried
out using integer arithmetic and are entirely reversible.
Consequently, the program can be used for either lossy or lossless
compression, with no special approach needed for the lossless case
(e.g. there is no need for a file of residuals.) Documentation on the
use of the programs and on the compression method is included in the doc
subdirectory.
These programs can compress 2-byte integer images in a small variety of
input formats (including FITS format, the most widely used format for
astronomical images.) The resulting compressed image file is
machine-independent and may be transferred between computers with
binary FTP. The source code is reasonably machine-independent,
although it has not been tested on a wide range of machines.
Installation instructions for Unix and VMS (VAX) machines are given in
the README files in the appropriate subdirectories. I would be very
interested to hear of any attempts to install this code on other kinds
of machines (e.g. MS-DOS, Macs.)
This directory has two files that contain the hcompress
image compression package:
hcompress.tar.Z - This is a compressed tar file,
which must be transferred using BINARY mode in FTP. You can extract the
files on a Unix system by doing zcat hcompress.tar.Z | tar xvf
This will create a directory called ./hcompress
(with some subdirectories.) The hcompress/aareadme file
gives installation information.
hcompress.bck - This is a VMS backup file, which must
be transferred using BINARY mode in FTP. When retrieved via anonymous
FTP, this file is created on your system with an incorrect blocksize.
To fix this, you must get 2 additional files: BACKUP.COM
(using ASCII mode) and MODIFY.EXE (using BINARY mode).
Then you can extract the files on a VMS system by doing
@BACKUP This will create a directory called
[.HCOMPRESS] (with some subdirectories.) The
[.HCOMPRESS]AAREADME file gives installation information.
This version (28 February 1994) corrects one bug in the decompression
from the previous version (20 April 1992). The effect of this bug was
only noticeable for lossless compression of odd-sized images (images
with dimensions that are not a power of 2.) The compressed files in
that case were correct, but the decompressed images could have pixel
values that differed by +/- 1 count compared to the original. If you
have any old compressed files, they will decompress properly using this
new version of the software. Only hinv.c has changed from
the previous version. I know of no other bugs that have been discovered
(but would welcome reports of such.)
Get Unix Compressed Tar File
![[Link]](http://www.stsci.edu/icons-html/orangeball.gif)
File hcompress.tar.Z
Get VMS Files
File hcompress.bck
File backup.com
File modify.exe
Copyright (c) 1994 Association of Universities for Research
in Astronomy. All rights reserved. Produced under National
Aeronautics and Space Administration Contract No. NAS5-26555.
STScI Home Page |
Software
Richard L. White, rlw@stsci.edu
1997 April 23